The present invention generally relates to the on-sight analysis of fluids produced from hydrocarbon producing wells. In particular, the present invention relates to devices and methods which provide real-time determination of the relative components of the fluids produced from hydrocarbon producing wells. In this invention, phase separation and the determination of the relative volumes of each are accomplished by utilizing a pair of vessels in series, where the first vessel eliminates substantially all of the gas phase from a sample by actuation of a piston within the vessel which delivers a degasified sample to the second vessel which further processes the sample and delivers a liquid phase sample to a cut analyzer which ascertains the relative percentages of water and oil in the sample (i.e., the “cut” or “water cut”).
An accurate real-time determination of water cut, when used in conjunction with other parameters such as oil gravity and gas liquid ratio, may be utilized to determine the real-time density of the produced fluid. Knowing the real-time density of the produced fluid may be utilized in conjunction with various devices, such as rod string load cells, to determine downhole pressures and real-time production rates. Knowing the water cut of individual wells on a real time basis also facilitates field wide reservoir analysis and management. For example, in a water flood operation, the detection of a sudden increase in a well's water cut provides useful information regarding the effectiveness of the flood.
The known cut analyzers are most accurate when analyzing a sample which does not have any free gas phase. Free gas in the sample typically results in under measurement of the water cut, so it is desirable to reduce or eliminate any free gas before analysis of the sample. Moreover, the presence of small, but unknown and variable amounts of entrained gases in the sample confound accurate fluid density measurement, which is critical to the extent that fluid density is an inputted variable for downhole monitoring and well diagnostics.
It is known that separation of the free gas phase from the liquid phase in a sample is desired prior to making a water cut determination is made. The American Society for Testing and Materials (“ASTM” and the American Petroleum Institute (“API”) have provided a Standard Test Method for Water and Sediment in Crude Oil by the Centrifuge Method (D 4007) which provides a laboratory procedure for making water cut determinations. This method is generally accurate because, among other reasons, free gas has already separated from the oil. However, this operation is time consuming and requires manual processing of the sample. It is not a method which may be replicated in the field for real time determination of water cut. Instead, a number of various water cut meters are utilized. These meters utilize various operating principles and hardware to make the water cut determination, such as dielectric measurements using radio or microwave frequencies, optical detectors for detecting near infrared wavelengths, and gamma ray based instruments. It is to be noted that the presently disclosed invention can utilize almost any of the types of devices for the eventual water cut determination. The presently disclosed invention improves the accuracy of these devices by providing a sample, on the fly at real-time conditions, where the sample is essentially gas free and, optionally, heated to API standard temperature for water cut determination.
The common automated mechanisms for gas separation typically require large separators which typically rely upon heat, gravity, mechanical flow dividers (such as baffles), and relatively long holding times to sufficiently separate the gas phase from the liquid phase to obtain an accurate determination of the cut. While portable skid units having relatively smaller separation vessels are known, the accuracy of the water cut determination can be adversely impacted by the relatively small separator size and short time for separation.